1. Field of the Invention
The present invention relates to low pressure plasma generation systems and, more particularly, to coil configurations for improving plasma uniformity in a plasma generation system.
2. Description of the Related Technology
Ionized gas or "plasma" may be used during processing and fabrication of semiconductor devices. Plasma is used to etch or remove material from semiconductor integrated circuit wafers. Plasma may also be used to deposit or sputter material onto integrated circuit wafers. Use of plasma gases in the fabrication of integrated circuits is widespread in the semiconductor manufacturing industry.
During fabrication, a semiconductor integrated circuit wafer may require materials to be added or removed, or selectively etched through a mask. To use plasma in the integrated circuit fabrication process, typically, a low pressure process gas is introduced into a process vessel chamber surrounding an integrated circuit wafer. The process vessel is used to maintain the low pressures required for the plasma and to serve as a structure for attachment of the energy source. The process gas is ionized into a plasma by the energy source, either before or after entering the chamber, and the plasma flows over the semiconductor integrated circuit wafer.
Ideally, uniformly ionized plasma would flow over the entire surface of the wafer. Any difference in the plasma ionization strength will cause uneven reaction characteristics along the surface of the wafer. Uneven reaction characteristics may cause problems when etching thin films associated with semiconductor manufacturing. Some of the problems created are etch rate non-uniformity across a substrate from edge to center, profile or line width variation across the substrate, and semiconductor device damage.
Plasma may be created from a low pressure process gas by inducing an electron flow which ionizes individual gas molecules through the transfer of kinetic energy through individual electron-gas molecule collisions. Various methods of inducing an electron flow in the process gas are well known to those skilled in the art. Typically, electrons are accelerated in an electric field such as one produced by radio frequency energy. Low frequencies (below 550 KHz), high frequencies (13.56 MHz), or microwaves (2.45 GHz).
Using microwave radio frequency energy to generate plasma has the advantage of more readily transferring energy to the process gas rather than to surrounding objects such as the walls of a process chamber or the semiconductor wafer. Another way of generating a plasma is with an electron cyclotron resonance (ECR) system. The ECR system requires a controlled magnetic field to induce circular electron energy into the process gas and not into the process chamber walls.
Other methods for improving the efficiency of plasma generation are magnetically enhanced plasma generation systems and inductively coupled electron acceleration, more commonly called inductively coupled plasma. Magnetically enhanced plasma systems use a constant magnetic field parallel to the integrated circuit wafer surface and a high frequency electric field perpendicular to the wafer surface. The combined magnetic and electric forces cause the electrons to follow a cycloidal path, thus, increasing the distance the electrons travel relative to the more direct straight path induced by the electric field alone. A major drawback in using a magnetic field to increase the electron travel distances is the requirement of a strong magnetic field which is both costly and difficult to maintain.
In the inductively coupled plasma process, the electrons also follow an extended circular path. Two techniques may be used to generate plasma by inductive coupling, both of which use alternating current to transfer energy to the gas by transformer coupling. The first technique utilizes a ferrite magnetic core to enhance transformer coupling between primary and secondary windings, and uses low frequencies, for example, below 550 KHz. The second technique uses a solenoid coil surrounding the gas to be ionized. This technique may use either low frequencies or high frequencies in the range of 13.56 MHz. Neither of these techniques provides a uniform plasma proximate and substantially parallel with the surface of an integrated circuit wafer.
U.S. Pat. No. 4,948,458 describes a method and apparatus for obtaining a more uniform and substantially parallel (planar) plasma for use during processing of integrated circuit wafers. The invention disclosed in this patent comprises an enclosure having an interior bounded at least in part by a radio frequency transparent window. A planar coil is disposed proximate to the window, and a radio frequency energy source is coupled through an impedance matching circuit to the coil. The planar coil radiates the radio frequency energy such that a planar magnetic field is induced in the interior of the enclosure. This planar magnetic field causes a circulating flow of electrons to be induced into the process gas.
The circulating flow allows the electrons to travel a path a much greater distance before striking the enclosure. The circulating electrons flow is substantially planar and has minimal kinetic energy in the non-planar direction. The planar coil is substantially parallel with a support surface. The support surface, therefore, is oriented substantially parallel to the circulating electron flow and is adapted to hold a semiconductor integrated circuit wafer during process fabrication. Thus, the support surface holds the semiconductor wafer substantially parallel to the electron flow.
The purpose of the invention disclosed in the above mentioned patent is to generally limit the wafer treatment to only the chemical interaction of the plasma species with the integrated circuit wafer. This is accomplished by minimizing the kinetic velocity of the plasma flux in the non-planar direction, thus reducing the kinetic impact on the wafer.
Referring to FIGS. 1 and 2, isometric and cross-sectional views of the prior art, respectively, are illustrated schematically. A plasma treatment system 10, for etching individual semiconductor wafers W, includes an enclosure 12 having an access port 14 formed in an upper wall 16. A radio frequency transparent window 18 is disposed below the upper wall 16 and extends across the access port 14. The window 18 is sealed to the wall 16 to define a vacuum-tight interior 19 of the enclosure 12.
A planar coil 20 is disposed within the access port 14 adjacent to the window 18. Coil 20 is formed as a spiral having a center tap 22 and an outer tap 24. The plane of the coil 20 is oriented parallel to both the window 18 and a support surface 13 upon which the wafer W is mounted. In this way, the coil 20 is able to produce a planar plasma within the interior 19 of the enclosure 12 which is parallel to the wafer W.
Referring now to FIGS. 1-3, the planar coil 20 is driven by a radio frequency (RF) generator 30. The output of the generator 30 is fed by a coaxial cable 32 to a matching circuit 34. The matching circuit 34 includes a primary coil 36 and a secondary loop 38 which may be mutually positioned to adjust the effective coupling of the circuit and allow for loading of the circuit at the frequency of operation. Conveniently, the primary coil 36 is mounted on a disk 40 which may be rotated about a vertical axis 42 in order to adjust the coupling therebetween.
A variable capacitor 44 is also provided in series with the secondary loop 38 in order to adjust the circuit resonant frequency with the frequency output of the RF generator 30. Impedance matching maximizes the efficiency of power transfer to the planar coil 20. An additional capacitor 46 is provided in the primary circuit in order to cancel part of the inductive reactance of coil 36 in the circuit.
Referring now to FIGS. 2 and 4, process gas is introduced into the interior 19 of the enclosure 12 through a port 50 formed through the side of the enclosure 12. The gas is introduced at a point which provides for distribution throughout the interior 19.
The flat spiral coil 20 may consist of equally spaced turns. Referring to FIG. 8, a graph representing test measurements of the current density versus position relative to the center of an equally spaced planar coil is illustrated. The graph of FIG. 8 illustrates maximum plasma density at or near the center of the equally spaced planar coil. This is also described in U.S. Pat. No. 4,948,458, column 6, lines 35 to 41.
Further tests, however, indicate that the equally spaced turns of the coil create a non-uniformity in the plasma generated. This is so because the side walls of the enclosure 12 cause more losses to the periphery of the plasma than toward the center of the plasma. Referring to FIG. 9, the current density versus position of an unmodified equally spaced spiral planar coil and a modified planar coil having unequal spacing of the turns, is illustrated. The current density 90 of the unmodified coil has a lower current density at the outer periphery 92 and 94 than does the modified coil current density 96. Thus, in contrast to the invention claimed in U.S. Pat. No. 4,948,458, a more uniform plasma over the entire surface of the semiconductor wafer requires not more but less RF power near the center of a planar spiral wound coil.